Engine combustion chamber valves, such as intake and exhaust valves, are typically spring biased toward a valve closed position. In many internal combustion engines, the engine valves may be opened and closed by fixed profile cams in the engine, i.e., by a valve train element. More specifically, valves may be opened or closed by one or more fixed lobes which may be an integral part of each of the cams. In some cases, the use of fixed profile cams may make it difficult to adjust the timings and/or amounts of engine valve lift. It may be desirable, however, to adjust valve opening times and/or lift for various engine operating conditions, such as positive power operation versus engine braking operation, or for different engine speeds during positive power and engine braking operation.
A method of adjusting valve timing and lift given a fixed cam profile, is to incorporate a “lost motion” device in the valve train linkage between the engine valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion dictated by a cam profile with a variable length mechanical, hydraulic, or other linkage means. The lost motion system may comprise a variable length device included in the valve train linkage between the cam and the engine valve. The lobe(s) on the cam may provide the “maximum” (longest dwell and greatest lift) motion needed for a range of engine operating conditions. When expanded fully, the variable length device (or lost motion system) may transmit all of the cam motion to the valve, and when contracted fully, transmit none or a reduced amount of cam motion to the valve. By selectively decreasing the length of the lost motion system, part or all of the motion imparted by the cam to the valve can be effectively subtracted or “lost.”
Hydraulic-based lost motion systems may provide a variable length device through use of a hydraulically extendable and retractable piston assembly. The length of the device is shortened when the piston is retracted into its hydraulic chamber, and the length of the device is increased when the piston is extended out of the hydraulic chamber. Alternatively, a hydraulic-based lost motion system may utilize a hydraulic circuit including a master piston and a slave piston which is selectively charged with hydraulic fluid to actuate an engine valve. The master and slave circuit may be depleted of hydraulic fluid when it is desired to “lose” the valve actuation motion input to the master piston, and the circuit may be charged with hydraulic fluid when it is desired to transfer the motion from the master piston to the slave piston and the engine valve. One or more hydraulic fluid control valves may be used to control the flow of hydraulic fluid into and out of the hydraulic chamber or hydraulic circuit.
One type of lost motion system, known as a Variable Valve Actuation (VVA) system, may provide multiple levels of lost motion. Hydraulic VVA systems may employ a high-speed control valve, referred to herein as a trigger valve, to rapidly change the amount of hydraulic fluid in the hydraulic chamber or circuit between the master and slave lost motion pistons. The trigger valve may be capable of rapidly draining hydraulic fluid from the chamber or circuit, thereby allowing the lost motion system to selectively lose a portion of an engine valve event to provide variable levels of valve actuation.
In the lost motion system of U.S. Pat. No. 5,680,841, an engine cam shaft may actuate a master piston which displaces fluid from its hydraulic chamber into a hydraulic chamber of a slave piston. The slave piston in turn acts on the engine valve to open it. The lost motion system may include a solenoid trigger valve in communication with the hydraulic circuit that includes the chambers of the master and slave pistons. The solenoid valve may be maintained in a closed position in order to retain hydraulic fluid in the circuit when the master piston is acted on by certain of the cam lobes. As long as the solenoid valve remains closed, the slave piston and the engine valve respond directly to the hydraulic fluid displaced by the motion of the master piston, which reciprocates in response to the cam lobe acting on it. When the solenoid is opened, the circuit may drain, and part or all of the hydraulic pressure generated by the master piston may be absorbed by the circuit rather than be applied to displace the slave piston and the engine valve.
Lost motion systems that utilize a master and slave circuit normally require that the master piston and the slave piston be provided in a common housing that can withstand the required high hydraulic pressures. Further, it may be desirable to place the master and slave pistons in close proximity to one another to avoid hydraulic compliance issues. Still further, it may be necessary to position the slave piston above the engine valve or valves that it actuates and to place the master piston such that it may receive valve actuation motion from a valve train element such as a rocker arm, cam, push tube, or the like. The foregoing requirements may present challenges to lost motion system designers due to the need to place the lost motion system into an already existing valve train in an engine compartment of limited size. Therefore, there is a need for a lost motion system which has a low profile relative to an existing valve train and which requires less engine compartment space.
Previous lost motion systems have typically not utilized high speed mechanisms to rapidly vary the length of the lost motion system, although the aforementioned '841 patent does contemplate the use of a high speed trigger valve. High speed lost motion systems in particular, are needed to provide Variable Valve Actuation (VVA). True variable valve actuation is contemplated as being sufficiently fast as to allow the lost motion system to assume more than one length within the duration of a single cam lobe motion, or at least during one cycle of the engine. By using a high speed mechanism to vary the length of the lost motion system, sufficiently precise control may be attained over valve actuation to enable more optimal valve actuation over a range of engine operating conditions. While many devices have been suggested for realizing various degrees of flexibility in valve timing and lift, lost motion hydraulic variable valve actuation is becoming recognized for superior potential in achieving the best mix of flexibility, low power consumption, and reliability.
Engine benefits from lost motion VVA systems can be achieved by creating complex cam profiles with extra lobes or bumps to provide auxiliary valve lifts in addition to the conventional main intake and exhaust events. Many unique modes of engine valve actuation may be produced by a VVA system that includes multi-lobed cams. The lost motion VVA system may be used to selectively cancel or activate any or all combinations of valve lifts possible from the assortment of lobes provided on the intake and exhaust cams. As a result, significant improvements may be made to both positive power and engine braking operation of the engine.
One particular engine valve actuation enabled by lost motion systems that is frequently desired by diesel engine manufacturers and operators is compression release engine braking operation. During engine braking, the exhaust valves may be selectively opened to convert, at least temporarily, an internal combustion engine into an air compressor. This air compressor effect may be accomplished by partially opening one or more exhaust valves near piston top dead center position for compression-release type braking, or by maintaining one or more exhaust valves in a partially open position for much or all of the piston motion for bleeder type braking. In doing so, the engine develops retarding horsepower to help slow the vehicle down. This can provide the operator increased control over the vehicle and substantially reduce wear on the service brakes of the vehicle. A properly designed and adjusted engine brake can develop retarding horsepower that is a substantial portion of the operating horsepower developed by the engine in positive power.
Another engine valve actuation that may be provided using a lost motion system is Exhaust Gas Recirculation (EGR). The braking power of an engine brake may be increased by selectively opening the exhaust and/or intake valves to carry out exhaust gas recirculation in combination with engine braking. Exhaust gas recirculation denotes the process of channeling exhaust gas back into the engine cylinder after it is exhausted out of the cylinder. The recirculation may take place through the intake valve or the exhaust valve. When the exhaust valve is used, for example, the exhaust valve may be opened briefly near bottom dead center on the intake stroke of the piston. Opening of the exhaust valve at this time may permit higher pressure exhaust gas from the exhaust manifold to circulate back into the cylinder. The recirculation of exhaust gas may increase the total gas mass in the cylinder at the time of the subsequent engine braking event, thereby increasing the braking effect realized.
Still another engine valve actuation that may be provided using a lost motion system is Early Exhaust Valve Opening (EEVO). Variation of the opening time of an exhaust valve during positive power can improve exhaust gas temperature control needed for emissions after treatment and/or provide turbocharger stimulation for improved transient torque. Therefore there is a need for a valve actuation system that is capable of providing variable levels of EEVO in response to engine operation conditions.
Used in conjunction with a properly designed lost motion system, trigger valves may provide true variable valve actuation responsive to a particular engine operation mode, engine speed, engine load, and/or other engine parameter that changes during operation. Trigger valves, however, require a sizable solenoid to operate at the required speeds for variable valve actuation. The combined size of the “valve” portion of the trigger valve and the solenoid may make it impractical to provide a dedicated trigger valve for each engine valve. The ability to provide variable valve actuation for each engine valve, however, would be advantageous. In particular, the ability to provide both compression release engine braking, exhaust gas recirculation, and/or EEVO using a given pair of engine exhaust valves that communicate with a common engine cylinder would be advantageous. Accordingly, there is a need for a lost motion system, and in particular a variable valve actuation lost motion system, that utilizes a single control valve, preferably a trigger valve, for control of more than one engine valve to provide compression release engine braking, exhaust gas recirculation, EEVO, and/or potentially other engine valve actuations.
Space and weight considerations are also of considerable concern to engine manufacturers. Accordingly it is desirable to reduce the size and weight of the engine subsystems responsible for valve actuation. Some embodiments of the present invention are directed towards meeting these needs by providing a compact master-slave piston and trigger valve combination for the lost motion VVA system. Applicants have discovered that some unexpected advantages may also be realized by reducing the size of the lost motion VVA system. As a result of reduction of the overall size of the system, the attendant hydraulic passages therein may be reduced in volume, thus improving hydraulic compliance.
Providing hydraulic fluid for initial operation of hydraulic-based VVA systems during engine start up also may be a concern of VVA designers and manufacturers. As some VVA systems may require hydraulic fluid immediately in order to provide basic engine valve actuations such as main intake and main exhaust events, it may be desirable to provide a VVA system which does not require any hydraulic fluid for main intake and main exhaust engine valve actuation.
Typically, engine valves are required to open and close very quickly, and therefore the valve return springs are generally relatively stiff. If left unchecked after a valve opening event, the valve return spring could cause the valve to impact its seat with sufficient force to cause damage to the valve and/or its seat. In valve actuation systems that use a valve lifter to follow a cam profile, the cam profile provides built-in valve closing velocity control. The cam profile may be formed so that the actuation lobe merges gently with cam base circle, which acts to decelerate the engine valve as it approaches its seat.
In some hydraulic lost motion systems, and in particular VVA hydraulic lost motion systems, rapid draining of fluid from the hydraulic circuit may prevent the valve from experiencing the valve seating provided by a cam profile. In some VVA systems, for example, an engine valve may be closed at an earlier time than that provided by the cam profile by rapidly releasing hydraulic fluid from the lost motion system. When fluid is released from the lost motion system, the valve return spring may cause the engine valve to “free fall” and impact the valve seat at an unacceptably high velocity. The engine valve may impact the valve seat with such force that it eventually erodes the valve or valve seat, or even cracks or breaks the valve. In such instances, engine valve seating velocity has been limited by controlling the release of hydraulic fluid from the lost motion system instead of by a fixed cam profile. Such devices have been referred to as “valve seating” devices or “valve catches.”
Valve seating devices may include hydraulic elements, and thus may need to be supported in a housing and require a supply of hydraulic fluid, yet at the same time fit within the packaging limits of a particular engine. The need to employ one or more valve seating devices thus adds complexity, cost, weight, and consumes limited engine compartment space. Further, the need to employ valve seating devices increases risk of engine failure or damage should the device ever fail or be denied of hydraulic fluid. Accordingly, it may be advantageous to provide a lost motion system, and particularly a VVA system, that does not require a valve seating device to gently seat an engine valve at the conclusion of an engine valve event.
Various embodiments of the present invention may meet one or more of the aforementioned needs and provide other benefits as well. Additional advantages of the invention are set forth, in part, in the description that follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.